Getting to Know Rocks

"We assended a steep high rockey part
of the Mountain high rocks and high pricipicies"

Basalt and Granites in Central Idaho

map the SW of Idaho consisting of basalt, the SE portion of granite, and the rest of other rocks

Based on data from the Idaho Department of Lands, Bureau of Mines and Geology, and United States Geological Survey, 1947.

Lewis and Clark and their men of the Corps of Discovery could not tell one rock from another. Very few people could in their day. None of those men had even the foggiest idea of the processes and events that shaped landscapes, put rocks where we now find them, and covered the rocks with soil. No one did in their time. Human knowledge of landscapes, rocks, soils, and geologic processes in general had not noticeably improved since the days of the Roman Empire. So it is no surprise that the journals of the Corps of Discovery do not inform us of the geologic fabric of the lands they crossed. However, our present knowledge, imperfect as it is, does inform our reading of their journals.

The received wisdom of the geographers of 200 years ago led Lewis and Clark to suppose that crossing the almost unknown western interior of North American would be a fairly simple matter. They set out expecting to follow the Missouri River west to its headwaters on a ridge that would resemble the long ridges of the Appalachians. They would cross the dividing ridge in a single portage, then descend the drainage on its western slope to the Pacific Ocean. They did not expect anything even remotely resembling the baffling maze of high mountains and broad valleys that comprise the northern Rocky Mountains.

Lewis and Clark blundered their way around, through, and across the mountains of western Montana to a remote headwater of the Salmon River at Lemhi Pass. After a short trek along the Salmon River in Idaho, they crossed Lost Trail Pass back into Montana, and followed the Bitterroot River downstream to the mouth of Lolo Creek. Then they took an Indian trail up Lolo Creek and across the Bitterroot Mountains in a condition of steadily deepening misery. The mountains were higher and steeper than any they had ever seen, much less imagined. Trees so densely covered most of the land that they only rarely glimpsed the country ahead. And they nearly starved for lack of game. That was one of the hardest passages of a very tough journey. And what was the cause of their travail? Rocks!

The whole geological story of the Bitterroot Mountains is very long and complicated—and somewhat speculative—but we'll keep it short and stick to the simple facts.


Funded in part by a grant from the Idaho Governor's Lewis and Clark Trail Committee